Advocate staff photo by HILARY SCHEINUK -- Patricia Collins prays during a prayer vigil in honor of Alton Sterling following an address from Louisiana Governor John Bel Edwards, Thursday, July 7, 2016, at the Living Faith Christian Center on Winbourne Avenue, in Baton Rouge, La.

Advocate staff photo by HILARY SCHEINUK -- Patricia Collins prays during a prayer vigil in honor of Alton Sterling following an address from Louisiana Governor John Bel Edwards, Thursday, July 7, 2016, at the Living Faith Christian Center on Winbourne Avenue, in Baton Rouge, La.

When police officers are prosecuted for killing someone in the line of duty, their actions are judged by a decades-old legal standard.

In deaths like the fatal shooting of Alton Sterling in front of a Baton Rouge convenience store, several experts say, the cases can be complicated and hard to prove.

Key to the decision to prosecute a case — or to jury deliberations if it goes to trial — is a 1985 U.S. Supreme Court ruling that helped set the boundary for lethal force by holding that an officer improperly shot a fleeing, unarmed burglar who he feared was going to escape. That ruling in Tennessee v. Garner and an 1989 case, Graham v. Connor, set the broad standard for police use of force, known as “objective reasonableness.”

David Klinger, professor of criminology and criminal justice at the University of Missouri-St. Louis, said the standard asks whether a reasonable officer would have used the same level of force in the same situation, without the benefit of hindsight. He said the totality of the circumstances leading up to the use of force plays an important role in making that determination.

“Deadly force is reasonable when a police officer has reason to believe his life or some other innocent’s life is in imminent danger,” Klinger said.

Officers showed up about 12:35 a.m. Tuesday at the Triple S Food Mart on North Foster Drive after an anonymous caller reported a man in a red shirt selling CDs outside the store had ordered someone off the property at gunpoint, police have said. One witness has said Sterling was not holding a gun during the confrontation with police.

Cell-phone videos of the incident don’t give a clear picture of his hands. One does show an officer removing something from Sterling’s pocket after the shooting. Baton Rouge police have said Sterling was armed.

Charles “Joe” Key, a retired Baltimore police lieutenant who is court-qualified expert on police and civilian use of force, said the legal standard of what an objectively reasonable officer would have done in the same situation can lead to confusion in the public.

He said people often focus on whether someone had a gun or other weapon in his or her hand, but that can miss the point. A 1996 FBI bulletin outlining that agency’s use-of-force policy says someone may pose an “imminent danger” even if not holding a weapon.

“Imminent does not mean immediate. What it means is the threat is pending,” Key said.

He added that split second-decisions officers make often play a role in that reasonableness analysis. Key said various studies on reaction time suggest that waiting to spot a weapon can be a mistake.

“If you wait until they’re pointing the gun at you before you use lethal force to stop the threat, you’re dead,” said Key, who has analyzed more than 1,000 lethal police-involved shootings.

But Klinger, who worked as a police officer in the 1980s, noted that if the officers believed Sterling had a weapon, it would be unusual for them to be as close to him as the two well-publicized cellphone videos appear to show. The officers confront Sterling, bring him to the ground, then kneel on him while he is pinned on his back.

“If there is a fight over a gun, over control of a gun, that is a very dangerous situation,” Klinger said.

While he suggested that the presence of bystanders in the area — evident from cellphone videos — may have been a factor in the officers’ decisions, Klinger also said it is hard to know what led them to get as close to Sterling as they did with the information available.

Ken Levy, an LSU criminal law professor, said that based on what he has seen in two videos of the killing, the shooting was not justified.

Levy said it appears the officers used the appropriate level of force initially to try to take down and arrest Sterling. But he believes they weren’t justified in pointing their guns at him once he was on the ground.

“Given the position Mr. Sterling was in, the officers could not have reasonably believed that he posed a threat to them,” Levy said.

P. Raymond Lamonica, an LSU law professor and former U.S. attorney in Baton Rouge, warned that federal criminal civil rights cases are tough to prove because of the high legal standard. Prosecutors have to show government officials, in this case the police, were acting under the “color of law” to deprive someone of his constitutional rights and that those officials acted with a “specific intent” to violate those constitutional rights.

Lamonica said a case where a racial epithet is spoken, for instance, might give prosecutors a good idea of what someone’s intent is.

“In excessive force cases where there’s no statement or anything like that, it’s more difficult. You have to prove willfulness,” he said.